Cables have long been used to transfer signals between computers and other electrical systems. Depending on an operating environment, a cable and/or its internal signals may be susceptible to power or voltage surges. For example, surges can be caused by lightning, static electricity, temporary ground differences, or even glitches in power supply sources. Failing to provide adequate protection against these transient voltage spikes (received through cables) typically results in a substantial amount of damage to electronic equipment every year.
Lightning strikes can cause temporary ground differences between two or more communicating devices, disrupting communication and causing circuit damage. For example, during normal operation, a remote device may send a communication signal referenced to its corresponding (remote) ground. Depending on a system's configuration, a local device may not be able to receive the communication signal unless its ground reference approximates that of the remote ground reference. However, during a lightning strike, the remote ground reference may substantially increase for a brief instant of time, thereby imparting excessive voltage onto a communication signal transmitted to the local device. If the local device is not properly protected, it may be damaged as a result of the excessive voltage imparted on the communication signal (caused by the lightning strike).
To protect against surges, a conventional approach involves dissipating power surges via suppression circuits that clamp an input voltage to a level that does not cause damage to a corresponding electronic circuit that receives the signal. Suppression circuits include transzorbs, zener diodes, arrestor devices such as metal oxide varistors, carbon blocks, thyristors, gas discharge tubes and the like. Typically, these clamping circuits are disposed directly on a circuit board including sensitive functional circuitry that needs protection against potentially damaging surges.
Another technique of protecting against surges involves the use of an optical isolator disposed in series with an electrical cable. Such a device converts an electrical signal potentially including transient voltage spikes to an optical signal. The optical signal is then converted back to an electrical signal and transmitted to a target device. Generally, optical devices support protocols such as RS-232.
A more sophisticated method of protecting against power surges is to employ detector circuits that detect the presence of a lightning storm during which a surge is likely to occur. In response to detecting such a dangerous condition, the detector circuits cause electronic equipment to be mechanically disconnected (via relays) from an external cable connection while the threat of the surge (e.g., a lightning storm) remains present. After the threat has subsided, the cable equipment is then reconnected to the cable again.